WASHINGTON, Nov 9 (Reuters) - The U.S. Environmental
Protection Agency said on Tuesday it has issued a subpoena to
Halliburton (HAL.N: Quote), demanding information about chemicals it
uses in a natural gas drilling technique called "fracking."

In September, the EPA had asked nine companies that
practice hydraulic fracturing, or fracking, to reveal the mix
of chemicals they use in the practice which is opposed by
environmental groups worried about its effect on drinking
water.

All but Halliburton provided the necessary information, the
EPA said.

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The EPA says it needs the data on fracking fluids to
complete its comprehensive study of the technique. During
fracking, companies inject millions of gallons of water, sand
and chemicals as far as two miles (3.2 km) underground to break
open fissures in the gas-bearing shale.

The EPA is slated to release preliminary results of the
study, commissioned by Congress, by the end of 2012.

Fracking has been around for decades, but companies have
expanded its use in recent years to extract abundant but
hard-to-access reserves of shale gas.

Homeowners near some shale field developments say fracking
has made their tap water toxic, and that in some cases
flammable gases have escaped through their spigots.

"UNREASONABLE DEMANDS"

Halliburton said it has been working with the EPA on its
request and had provided some 5,000 pages of documents as of
last week. The company said it was meeting with the EPA "in
order to help narrow the focus of their unreasonable demands"
that could require them to prepare about 50,000 spreadsheets.

An EPA official said Halliburton had submitted publicly
available forms that describe the general hazards associated
with various chemical mixes.

"While these are helpful, they do not identify each and
every chemical in a mixture, which is precisely the sort of
information EPA needs to understand the risks these ...
mixtures may pose to drinking water supplies," the official
said.

The EPA had also asked all of the companies for standard
operating procedures that govern the choice and amount of
chemicals and water used in fracking jobs, along with locations
of every fracking job undertaken in the previous year or
planned for the next. Halliburton has not given that
information.

In 2005, the administration of George W. Bush passed the
so-called "Halliburton Loophole" which exempts fracking
companies from regulation under the Safe Drinking Water Act.

Environmentalists said Halliburton should be required to
reveal the chemicals.

"As if being exempt from the Safe Drinking Water Act wasn't
enough, Halliburton is now refusing to comply with EPA requests
designed to protect public health," said Megan Klein, an
associate lawyer with Earthjustice.